An international group of physicists led by Prof Vladan Vuletic of MIT has developed a novel technique that can successfully entangle 3,000 atoms using one photon.
NIST-F1 Cesium fountain atomic clock, serving as the U.S. time and frequency standard. Image credit: NIST.
The new technique, described in the journal Nature, provides a realistic method to generate large ensembles of entangled atoms, which are key components for realizing more-precise atomic clocks.
Today’s best atomic clocks are based on the natural oscillations within a cloud of trapped atoms. As the atoms oscillate, they act as a pendulum, keeping steady time. A laser beam within the clock, directed through the cloud of atoms, can detect the atoms’ vibrations, which ultimately determine the length of a single second.
The accuracy of atomic clocks improves as more and more atoms oscillate in a cloud. Precision of atomic clocks is proportional to the square root of the number of atoms. For example, a clock with 9 times more atoms would only be 3 times as accurate. If these same atoms were entangled, a clock’s precision could be directly proportional to the number of atoms – in this case, 9 times as accurate.
The larger the number of entangled particles, then, the better an atomic clock’s timekeeping.
Entanglement is a curious phenomenon – as the theory goes, two or more particles may be correlated in such a way that any change to one will simultaneously change the other, no matter how far apart they may be. For instance, if one atom in an entangled pair were somehow made to spin clockwise, the other atom would instantly be known to spin counterclockwise, even though the two may be physically separated by thousands of km.
Physicists have so far been able to entangle large groups of atoms, although most attempts have only generated entanglement between pairs in a group. Only one group has successfully entangled 100 atoms.
Now, Prof Vuletic’s team has successfully created a mutual entanglement among 3,000 atoms, virtually all the atoms in the ensemble, using very weak laser light – down to pulses containing a single photon.
“You can make the argument that a single photon cannot possibly change the state of 3,000 atoms, but this one photon does – it builds up correlations that you didn’t have before. We have basically opened up a new class of entangled states we can make, but there are many more new classes to be explored,” said Prof Vuletic, who is the senior author of the Nature paper.
Prof Vuletic and his colleagues from MIT and the University of Belgrade, Serbia, first cooled a cloud of atoms, then trapped them in a laser trap, and sent a weak laser pulse through the cloud.
They then set up a detector to look for a particular photon within the beam.
“If a photon has passed through the atom cloud without event, its polarization would remain the same. If, however, a photon has interacted with the atoms, its polarization rotates just slightly – a sign that it was affected by quantum noise in the ensemble of spinning atoms, with the noise being the difference in the number of atoms spinning clockwise and counterclockwise,” the scientists said.
“Every now and then, we observe an outgoing photon whose electric field oscillates in a direction perpendicular to that of the incoming photons. When we detect such a photon, we know that must have been caused by the atomic ensemble, and surprisingly enough, that detection generates a very strongly entangled state of the atoms,” Prof Vuletic said.
The physicists are currently using the single-photon detection technique to build a state-of-the-art atomic clock that they hope will overcome what’s known as the ‘standard quantum limit’ – a limit to how accurate measurements can be in quantum systems.
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Robert McConnell et al. 2015. Entanglement with negative Wigner function of almost 3,000 atoms heralded by one photon. Nature 519, 439-442; doi: 10.1038/nature14293
